Ionic conductors have a long history and so far various types of ionic conductors have been developed. Specifically, for example, zirconium oxide or cerium oxide which is an oxide ion conductor has been used as an ionic conductor for fuel cells and gas sensors.
As for proton conductors, SrCe1-xMxO3, CaZr1-xMxO3 and SrZr1-xMxO3 (M is a trivalent element) have been reported.
In addition, BaCe1-xMxO3 (M is a trivalent element), which is an oxide of barium and cerium, has been reported as an ionic conductor which conducts oxide ions and protons simultaneously. In particular, when Gd is used as M and x is adjusted to 0.16 to 0.23, high conductivity can be obtained (e.g., Patent Document 1).
Although many other ionic conductors have been found, few of them satisfy both electric conductance and reliability and have been put into practical use. At present, only zirconia is used for oxygen sensors and SrCe1-xMxO3 and CaZr1-xMxO3 are used to detect the hydrogen concentration in a melting furnace.
However, SrCe1-xMxO3 and CaZr1-xMxO3, in particular, work in limited environment and are not sufficiently reliable. For example, when these materials are boiled in water, they are decomposed in about 1 to 100 hours. Further, change which seems to be attributable to decomposition of the above materials is observed at 85° C. and 85% RH.
Moreover, a mixed ionic conductor BaCe1-xMxO3 also dissolves in boiling water or is deteriorated in high humidity. At present, proton conductors which are composed of a perovskite oxide and stable in high humidity hardly exist.
Further, Patent Document 2 proposes BaZr1-x-yCexMyO3, which is a perovskite oxide usable as an ionic conductor in view of stability in boiling water. However, BaZr1-x-yCexMyO3 has low ion conductivity compared to BaCe1-xMxO3 (M is a trivalent element).
In addition, since an ionic conductor of solid oxide is usually used at high temperatures, such a conductor is required to be resistant to thermal shock. Further, since such a proton conductor is often used in reducing atmosphere, it is also required to have reduction resistance.
As described above, to provide a highly reliable device, ionic conductors must have high ion conductivity and stability. However, although current ionic conductors composed of perovskite oxide have high ion conductivity, they have poor stability including moisture resistance, reduction resistance and thermal shock resistance. In particular, indium oxide has poor stability as they tend to be reduced in reducing atmosphere at high temperatures.
Patent Document 1: JP 03-176700 A
Patent Document 2: JP 2000-302550 A